This invention relates to a cordierite body which has a low coefficient of thermal expansion (CTE). The invention relates also to a method for fabricating the body which makes use of a composition of magnesium oxide, aluminum oxide and silica containing material without clay and without talc. More particularly, the body is formed by extrusion. Still more particularly, the body has a honeycomb structure. A cordierite body having a honeycomb structure is especially suitable for but not limited to use as a substrate for a catalytic converter for automobiles.
Extruded cordierite honeycombs are manufactured for use as substrates to support catalytically active components for catalytic converters on automobiles. Use of cordierite is favorable in this application because of its good thermal shock resistance. The thermal shock resistance (TSR) is inversely proportional to the coefficient of thermal expansion. That is, honeycombs with a low thermal expansion have good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application.
Up to this time cordierite has been made typically with starting raw materials of clay, talc and alumina. U.S. Pat. No. 3,885,977 relates to extruded honeycombed monolithic fired ceramics made with these raw materials. In this patent it is stated that the orientation of the cordierite crystals in the fired body as measured by the I ratio, comes about because cordierite forms on oriented clay particles which are aligned by the extrusion process resulting in the desired low CTE. Therefore it was believed that the clay was necessary to form low expansion cordierite.
U.S. Pat. No. 4,280,845 relates to cordierite ceramic fabricated from a mixture of magnesia containing material, alumina, and silica raw materials, using clay and talc predominately. The magnesia containing raw material has a particle size of 5-150 microns. Raw material components yielding magnesium oxide, aluminum oxide, and silica without clay or talc are mentioned. However, the coefficient of thermal expansion in these cases is no less than about 10.5.times.10.sup.-7 /.degree. C. from room temperature to about 1000.degree. C. The lowest CTE cordierite bodies were obtained with batches which contained clay and talc. The firing rate above 1100.degree. C. is high. Therefore it is believed that a complete reaction is not attained. It is well understood that the firing time and temperature must be sufficient to achieve a complete reaction. The results of an incomplete reaction are that the CTE is high due to the presence of phases other than cordierite.
In U.S. Pat. No. 4,434,117 it has been further suggested that the use of a platy talc without clay is a necessary and sufficient condition in order to achieve a cordierite body with a low coefficient of thermal expansion when this platy talc is combined with other non-clay components to obtain the cordierite composition.